Using the following characters ONLY

abcdefghijklmopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789
~!@#$%^&()_+`-=[];',.{}

I need a decent algorithm to encrypt a textbox string which holds a file name and extension that must not be readable by human eyes i.e.

"File.txt"

"Gd7&.@0x"

I also have a textbox where the user can set a password so that the filename and extension can also be decrypted back to the original file name.

Offers and time required please.